Dispersions of hard, non-compliant particulates may be used in various technologies, including such areas as abrasive coatings, inks, paints, color proofing, etc. One area where dispersions of hard, non-compliant particles are used is magnetic recording media, such as audio tapes, video tapes, data storage tapes, or computer diskettes. In making such magnetic recording media, typically a substrate is coated with magnetic pigment particles and polymeric binder dispersed in a solvent. The solvent is removed by drying leaving a layer of magnetic recording material.
Current compounding technology for the processing of magnetic pigment dispersions employs media mills, such as a ball mill, a sand mill, or an attritor. Media mills achieve acceptable magnetic pigment dispersions by subjecting the mixture to high intensity microshearing which is essential for breaking down agglomerations of the pigment particles. However, these media mill processing systems suffer from several disadvantages including media wear product contamination, e.g. sand particles in the dispersion. Furthermore, the processing rate for media mills is limited. If the flow-through rate in a media mill is increased, uneven grinding and dispersion occurs and much of the material leaves the system without being sufficiently processed. It would be desirable to avoid these disadvantages of media mill processing by using high pressure systems like homogenizers and emulsifiers.
Homogenizers and emulsifiers generally function by forcing a premix of solids and liquids to collide against a surface or against itself. Unfortunately, processing hard, non-compliant particle dispersions in high pressure emulsifiers has been difficult due to abrasiveness of the particles and the relatively large size of agglomerated structures which could plug the narrow gaps through which the mixture was forced. To avoid this clogging, U.S. Pat. Nos. 4,533,254 and 4,908,154 require filtration or preprocessing to reduce the size of the pigment and to ensure good dispersion of the pigment prior to use of a high pressure homogenizer or emulsifier.
In addition, the abrasiveness of magnetic pigment causes rapid wear on the impingement chambers. Difficulty in monitoring the prior art homogenizers or emulsifiers for wear or clogging and inability to inexpensively and quickly replace worn parts have been major obstacles to using high pressure devices.
Finally, prior art homogenizers or emulsifiers generally do not exceed operating pressures of 30,000 pounds per square inch (205 MPa), and, as a result, the amount of processing energy that could be applied to the mixture is limited. Note, however, that Japanese applications 05098192 and JP0509188 to Dainippon Ink & Chemical, which teach how to attain a colloidal suspension of a polymer, indicate a preference, however, for jet impingement pressures in the range of 1400-140,000 psi (9.8-980 MPa).
The prior art also teaches that a preconditioning process may advantageously be used prior to media milling during the preparation of magnetic pigment dispersions. This preconditioning process is usually carried out on a complete charge of the magnetic pigment, at least a portion of the solvent, and, optionally, a portion or all of the polymeric binder and other additives. Preconditioning improves subsequent handling and processing (milling, etc.) by promoting initial wetting of the pigments by surfactants, polymers, etc., and by displacing air from the surface of the particles. High speed mixers, homogenizers, kneaders, and planetary mixers have been used for this process.
A system where hard particles are forced through a series of decreasing size orifices has been used in the past to manufacture magnetic pigment dispersions.